The importance of radio frequency identification (RFID) systems is still increasing particularly in the service sector, in the field of logistics, in the field of commerce and in the field of industrial production. Thus, contactless identification systems or RFID systems are implemented more and more in these fields and will probably substitute barcode systems in the near future.
In particular, such systems are suitable for wireless data transmission in a fast manner and without cable connections and consist of at least one base station, in particular a reader device and one or more transponders. Different types of transponders are known from the art, in particular depending on their kind of energy supply. Some of said transponders do not have a power supply of their own and are therefore called passive transponders. Passive transponders take the electrical energy required for their own power supply from an electromagnetic field provided by the base station.
For data transmission a transponder can be inductively coupled to said base station and comprises an electronic circuit, usually a microchip, connected to a large area coil that functions as an antenna. If the transponder is moved into an electromagnetic field provided by the base station, a current is induced in the large area coil of the transponder and the electronic circuit of the transponder is powered up.
In general, an inductive coupling is used to transmit data from the transponder to the base station and vice versa using an electromagnetic carrier wave provided by the base station. To this end, an ultra high-frequency (UHF) carrier wave signal or a high-frequency (HF) carrier wave signal is generated by the base station, which is received by the antenna of the transponder. Said electromagnetic carrier wave signal may have a frequency of for instance approximately 13 MHz (HF) or approximately 800 to 900 MHz (UHF).
For Near Field Communication (NFC) reflection characteristics of the antenna can be influenced by altering a load connected to the antenna of the transponder. In order to transmit data from the transponder to the base station, for example a load resistor, being connected in parallel with the coil is switched on and off in time with a data stream to be transmitted via an air interface. An amplitude of a reflected portion of electromagnetic carrier wave can thus be modulated (so called load-modulation).
Contrary to that, far field communication systems are based on a radar principle. A portion of the incoming power of said electromagnetic carrier wave signal produced by the base station is reflected by the antenna of the transponder and returned to the base station. This is also referred to as “backscatter” principle.
Additionally, subcarrier modulation techniques can be applied in combination with said load modulation. In said techniques, a data signal is modulated on a subcarrier signal and in accordance with that, the load of the transponder is switched on and off. The subcarrier signal may have a frequency of 848 kHz, for instance. In the base station the backscattered signal is received by an antenna and the transmitted data signal is then restored based on an evaluation of the backscattered signal.
A data signal to be transmitted consists of a sequence of symbols, each of which being coded according to a coding scheme and transmitted by the transponder within a predefined time interval or a what is called symbol duration. Possible coding schemes for such RFID systems are for instance Manchester, modified Miller, Non Return to Zero, Pulse Interval Encoding, etc. Other, higher order modulation and/or coding schemes can be used as well.
Using load modulation based on a coded subcarrier signal, an enhancement of a transmission rate is limited by the frequency of the subcarrier signal. Based on the subcarrier frequency used, a symbol duration is given to which a specific coding is applied. In conventional RFID systems, numerous higher order coding methods are applied. One of these methods is characterized by the fact, that within a predefined length of a symbol duration of an encoded symbol, a position of a transition from a High levelHigh level to a Low level of an encoded signal within the symbol duration defines one symbol of a coded character set. To form different symbols, the transition from the High level to a Low levelLow level of the encoded signal is shifted from sample to sample, each sample characterizing a unique symbol of the encoded character set. For a detection of the transition from the High- to the Low level at the base station an integrator may be used. Nevertheless it is to be mentioned that the number of symbols within a symbol duration is limited—without enhancing the complexity tremendously, resulting in still very low data rates.
From WO-03088499 A1 is known a method of coding data communication between a base station and an RFID transponder, wherein the data are encoded by an active time period of sub-carrier modulation and an inactive time period of no modulation. The modulation characteristics of the modulation such as phase and/or frequency are varied during the active time period in order to increase the data rate to be transmitted from the transponder to the base station.
US 2004/0257203 A1 describes a data encoding and decoding method within an RFID system having an improved error detection as well as error correction.
US 2005/0207391 A1 discloses a method for wireless data transmission between a base station and at least one transponder, in which an electromagnetic carrier wave is emitted by the base station and symbols are transmitted from a given transponder to the base station by modulating and backscattering the electromagnetic carrier waves. The transmission rate can be enhanced by changing the modulation state synchronously to a synchronization marker transmitted by the base station.
Significantly higher data rates will be needed for future applications. Therefore, new coding strategies and concepts of data transmission are needed in the technical field of RFID. The conventional technologies will not be able to fulfill these needs, except at the price of extremely difficult signal processing.